The ESIS-Technical Committee 9 on Concrete was established in 1990 and
has met seven times. A proposal was put to European and extra-European
laboratories entitled "Scale effects and transitional failure phenomena
of reinforced concrete beams in flexure" which lead to several positive
responses. The central topic discussed by the committee was that of the
minimum reinforcement in concrete members. The minimum amount of
reinforcement is defined as that for which "peak load at first concrete
cracking" and "ultimate load after steel yielding" are equal. In this
way, any brittle behaviour is avoided as well as any localized failure,
if the member is not over-reinforced. In other words, there is a
reinforcement percentage range, depending on the size-scale, within
which the plastic limit analysis may be applied with its static and
kinematic theorems. Carpinteri, Ferro, Bosco and El-Katieb propose a
LEFM model, according to which reinforcement reactions are applied
directly on the crack surfaces and a compatibility condition is locally
imposed on the crack opening displacement in correspondence with the
reinforcement. The theoretical model is found to provide a satisfactory
estimate of the minimum percentage of reinforcement that depends on the
scale and enables the element in flexure to prevent brittle failure.